Method for operating an ion beam therapy system by monitoring the distribution of the radiation dose

ABSTRACT

A method for operation of an ion beam therapy system that comprises a grid scanner device, arranged in a beam guidance system ( 6, 8 ), having vertical deflection means ( 13 ) and horizontal deflection means ( 14 ) for the vertical and horizontal deflection of a treatment beam ( 11 ) perpendicular to its beam direction, with the result that the treatment beam ( 11 ) is deflected by the grid scanner device to an isocentre ( 10 ) of the irradiation site and scans a specific area surrounding the isocentre ( 10 ) using a specific radiation dose. Both the depth dose distribution and the transverse dose distribution of the grid scanner device ( 13, 14 ) at various positions in the region of the isocentre ( 10 ) are measured and evaluated, it being concluded that the radiation dose distribution is adequately homogeneous if the degree of variation in the radiation dose values measured at the individual positions does not exceed a specific tolerance limit value.

The present invention relates to a method for the operation an ion beamtherapy system especially operated with heavy ions.

Ion beam therapy systems are preferably used in the treatment oftumours. An advantage of such systems is that, on irradiation of atarget object (target), the major portion of the energy of the ion beamis transferred to the target, while only a small amount of energy istransferred to healthy tissue. A relatively high dose of radiation cantherefore be used to treat a patient. X-rays, on the other hand,transfer their energy equally to the target and to healthy tissue, sothat for health reasons, for the protection of the patient, it is notpossible to use a high dose of radiation.

There is known from U.S. Pat. No. 4,870,287, for example, an ion beamtherapy system in which there are generated from a proton source protonbeams of which the protons can be delivered to various treatment orirradiation sites by an accelerator device. Provided at each treatmentsite is a rotating cradle having a patient couch so that the patient canbe irradiated with the proton beam at different angles of irradiation.While the patient is spatially located in a fixed position inside therotating cradle, the rotating cradle revolves round the body of thepatient in order to focus the treatment beams at various angles ofirradiation onto the target located in the isocentre of the rotatingcradle. The accelerator device comprises a combination of a linearaccelerator (LINAC) and a so-called synchrotron ring.

In H.F. Weehuizen et al. CLOSED LOOP CONTROL OF A CYCLOTRON BEAM FORPROTON THERAPY, KEK Proceedings 97-17, January 1998, a method ofstabilising the proton beam in proton beam therapy systems is proposedin which the treatment beam is actively so controlled that it is locatedon the centre line of the corresponding beam delivery system at twomeasurement points spaced from each other in the longitudinal direction.The first measurement point is located between a pair of deflectionmagnets and is formed by a multi-wire ionisation chamber. Depending onthe actual value of the beam position delivered from that multi-wireionisation chamber relative to the centre point of the beam path, a PIcontrol is generated by further deflection magnets arranged upstreamfrom the first-mentioned pair of deflection magnets. The secondmeasurement point is located just upstream of the isocentre and isformed by an ionisation chamber which is divided into four quadrants.Depending on the actual position value of that ionisation chamber, againPI control signals are generated, but those control signals are intendedfor the first-mentioned deflection magnets. Such a control arrangementis said to render possible both angle stability in terms of the centreline of the beam delivery system and lateral position stability of theproton beam.

When, however, heavy ion irradiation is carried out, that is to sayirradiation with ions that are heavier than protons, large and heavydevices are necessary, with the result that there is a tendency to avoidthe use of rotating cradles and instead move the patient or the patientcouch. Corresponding therapy systems are described, for example, in E.Pedroni: Beam Delivery, Proc. 1^(st) Int. Symposium on Hadrontherapy,Como, Italy, Oct. 18-21, 1993, page 434. Such systems are accordinglyeccentric systems.

Since, however, mainly isocentric systems are preferred by oncologists,a heavy ion beam therapy system was proposed in which, although rotatingcradles are used at the treatment sites, the radii of the rotatingcradles can be reduced by virtue of the treatment beam delivered to eachrotating cradle horizontally along its axis of rotation being so guidedby means of suitable magnet and optics arrangements that, for theirradiation of a target, the beam is first of all directed away from theaxis of rotation and later crosses the axis of rotation again in theisocentre. There is provided for the irradiation of the target a gridscanner, which comprises vertical deflection means and horizontaldeflection means, each of which deflects the treatment beamsperpendicular to the beam axis, with the result that an area surroundingthe target is scanned by the treatment beams. Such a system thusessentially provides beam guidance in only one plane of the rotatingcradle.

Since a high level of operational safety and operational stability interms of the treatment beam is always necessary in ion beam therapysystems, a monitoring device for monitoring the treatment beam deliveredby the grid scanner is provided in the afore-described heavy ion beamtherapy system. This monitoring device is arranged between the lastdeflection magnet of the above-mentioned magnet arrangement and theisocentre, and can comprise ionisation chambers for monitoring theparticle flow and multi-wire chambers for monitoring the beam positionand the beam width.

For safety reasons, various DIN standards have to be observed in theoperation of medical electron accelerators. Those standards areconcerned on the one hand with the inspection test, that is, theinspection of the readiness for operation, and on the other hand withthe consistency test, that is, examination of operational stability, ofthe system. For ion beam therapy systems, especially for heavy ion beamtherapy systems, safety standards of that kind developed specificallyfor such systems are not yet known, but there is still a need, in ionbeam therapy systems too, for as high as possible a level of operationalsafety and operational stability.

The problem underlying the present invention is therefore to propose amethod for the operation of an ion beam therapy system, wherein adequateoperational safety and operational stability with respect to theperformance of irradiation is ensured. The method shall at the same timebe suitable especially for use with heavy ions.

The problem is solved in accordance with the present invention by amethod having the features of claim 1. The dependent claims each definepreferred and advantageous embodiments of the present invention.

The present invention relates to a method for the operation of an ionbeam therapy system which comprises a grid scanner device, arranged in abeam guidance system, having vertical deflecting means and horizontaldeflecting means for vertical and horizontal deflection of a treatmentbeam perpendicular to its beam direction so that the treatment beam isdeflected by the grid scanner device to an isocentre of the irradiationsite and scans a specific area surrounding the isocentre with a specificradiation dose. Preferably, both the depth distribution of the dose andthe transverse distribution of the dose of the grid scanner device atvarious positions in the region of the isocentre are measured andevaluated, it being concluded that the radiation dose distribution isadequately homogeneous if the degree of variation in the radiation dosevalues measured at the individual positions does not exceed a specifictolerance limit value.

It is furthermore proposed to monitor, inter alia, the variation overtime in calibration factors of monitoring means that are used formonitoring specific beam parameters of the treatment beam, the influenceof the particle fluence or particle flow of the treatment beam on thosecalibration factors, the dependence of the calibration factors on thebeam position of the treatment beam and the consistency of the treatmentdose, in each case appropriate tolerance limits being defined asintervention thresholds.

The present invention defines a comprehensive checking plan for ion beamtherapy systems. The present invention accordingly renders possible aclear improvement in the operational stability and operational safety ofthe ion beam therapy system, it being possible to perform the particularchecking aspects of the checking plan in the sense of an inspection testand/or a consistency test of the ion beam therapy system. This isespecially concerned with the checking of features of the ion beamtherapy system that are concerned with the grid scanning procedure andthe dosimetry region.

The invention is described in the following by way of a preferredexemplary embodiment with reference to the accompanying drawings.

FIG. 1 is a simplified schematic representation of an accelerationdevice used in the present ion beam therapy system;

FIG. 2 is a view of a rotating cradle used in the present ion beamtherapy system; and

FIGS. 3A and 3B and FIGS. 4A and 4B are, in each case, diagrams thatillustrate the advantageous effects of beam position control measures inthe ion beam therapy system shown in FIGS. 1 and 2.

An ion beam therapy system, on which the present invention is based, isgenerally used in hospital buildings that are subdivided into a medicalzone and an accelerator zone. A plurality of treatment or irradiationsites are provided for the treatment of patients. The supervisorycontrol system of the ion beam therapy system comprises a plurality ofsupervisory control rooms, it being possible for technical supervisorycontrol rooms to be provided for the individual treatment sites and fora main supervisory control room to be provided for the acceleratordevice. Laboratories for the dosimetry or for accelerator maintenance,or a PET device (positron emitter tomograph), can also be accommodatedin the building. In addition, energy supply devices (especially for theaccelerator device and the irradiation system) and cooling devices areprovided. In order to ensure an adequate shielding action, theindividual treatment rooms are bounded by thick walls and ceilings,which consist, for example, of concrete of a thickness of 2 m.

Since the basic structure of the ion beam therapy system is essentiallynot the theme of the present invention, only a brief description isgiven here.

The ion beam therapy system comprises an injection system, which isshown in simplified form in FIG. 1 together with the accelerator devicealready mentioned hereinbefore.

The injection system comprises ion sources 1 of which the radiation isin each case fed, by way of low energy beam guidance channels with anarrangement of spectrometer magnets and quadrupoles, to a switchingmagnet that guides the radiation, inter alia by way of a furtherquadrupole arrangement and by way of a chopper arrangement provided forpulse formation, to a linear accelerator 2 (LINAC).

In the present exemplary embodiment there shall be used exclusively ²C²+ions, which are stripped to ²C⁶+ in the beam guide between the linearaccelerator 3 and the synchrotron ring 5. For that purpose, a stripper 3is provided downstream of the linear accelerator 2. Owing to theirphysical and biological properties, those carbon ions have proved to bevery effective in the treatment of tumours and have the advantages of ahigh physical selectivity and a high biological effectiveness and, inaddition, offer the possibility of verification of the irradiation withthe aid of a positron emitter tomograph (PET). By suitable selection ofthe carbon ions, the biological effectiveness can be controlled in sucha manner that it is low in the plateau region of the Bragg curve andhigh in the region of the Bragg peak. Consequently, the target or thetumour can be treated with a comparatively high dose while the dose forthe surrounding healthy tissue is minimised.

In order to ensure the use and acceleration exclusively of the type ofion intended, a charge spectrum of the beam present is recorded andevaluated in the high-charge injection system.

By comparison of the recorded charge spectrum with a reference spectrum,undesired ions or irregularities can be detected and appropriatemeasures taken. This check can be carried out, for example, with eachinitialisation of an ion source 1.

The linear accelerator 2 is used for the initial acceleration of theions fed to it, those ions then being delivered by an injection line 4to the synchrotron 5. The injection line 4 comprises, in addition to thestripper 3 already mentioned, a further chopper arrangement for preciseshaping of the injection pulses, dipole magnets for charge analysis,quadrupoles for adapting the radiation to the receiving capacity of thesynchrotron 5 etc.

The injection system, which comprises inter alia the ion sources 1, thelow energy beam guidance channels, the linear accelerator 2 (LINAC), thestripper 3 and the injection line 4, thus has the task of generating andanalysing ion beams having desired particles, monitoring thecontamination of the ion beams and controlling the ion beam intensity,accelerating the ions to a particular injection energy and determiningthe pulse length of the pulses injected into the synchrotron ring 5.

The synchrotron ring 5 serves for the final acceleration of the ions fedto it to a determined energy and comprises, for example, a plurality ofdeflection magnets, quadrupoles and sextupoles. In the exemplaryembodiment shown in FIG. 1, there are provided, for example, sixdeflection magnets each having a deflection angle of 60°. Arrangedinside the synchrotron 5 is a cooling means (not shown). By means ofrepeated injection cycles, the injected ions are accelerated from anenergy in the region of a few MeV/u to an energy of, for example, morethan 400 MeV/u. The treatment beam accelerated in that manner isextracted at a particular point in the synchrotron by way of a highenergy beam guidance channel 6 and delivered to the individual treatmentsites.

Although the horizontal and vertical broadening of the beam at thetreatment site is generally variable, the demands for an “ideal”symmetrical and stable beam shape at the treatment site can besubstantially taken care of by a suitable adjustment of the beam opticsin the beam guidance channels.

The high energy beam guidance channel 6 comprises quadropole lenses,deflection magnets, beam analysis devices etc. In addition, a furtherchopper can be arranged downstream from the extraction point in thesynchrotron 5, which in emergencies is used to interrupt the beamsupply. In addition, a routine interruption of the extraction procedure,which serves to decouple the treatment beam from the synchrotron 5, canbe provided after each grid scan section.

FIG. 2 is a perspective view of one of the rotating cradles 8, which areeach provided at one of the treatment sites to which the treatment beamis delivered by way of the afore-described high energy beam guidancechannel 6. The rotating cradle 8 rotates about a particular axis ofrotation, while a patient to be treated lies on a patient couch 9 inlocally fixed orientation and alignment. The region of the patient'sbody to be treated is in that arrangement located in the isocentre 10 ofthe treatment beam, the isocentre being defined as the point ofintersection between the central beam 11 of the grid scanner describedin detail hereinafter and an axis of rotation of the patient couch 9.

As can be seen from FIG. 2, the high energy beam guidance channel 6 isso constructed that the treatment beam, after entry into the rotatingcradle 8, is deflected several times in one plane. There are providedfor that purpose a plurality of quadrupole lenses 12 and dipole magnets7, the first two dipole magnets 7 having identical deflection angles,for example 42°, and being arranged opposite each other, whilst the lastdipole magnet 7 is a deflection magnet having a deflection angle of 90°,with the result that after the treatment beam 11 has entered into therotating cradle 8 it is first deflected laterally out of the axis ofrotation of the rotating cradle 8 and then guided parallel to the axisof rotation of the rotating cradle 8 in order subsequently to leave thelast deflection magnet 7, via a beam outlet aperture, at an angle of 90°with respect to the patient couch 9.

In the exemplary embodiment shown in FIG. 2, the grid scannerarrangement provided in the present ion beam therapy system is arrangedbetween the last quadrupole lens 12 and the last deflection magnet 7 ofthe rotating cradle 8 and comprises at least one horizontal grid scannermagnet 13 and at least one vertical grid scanner magnet 14. The gridscanner magnets 13 and 14 each deflect the ion beam 11 perpendicular tothe beam axis 11 either horizontally or vertically, with the result thatthe ion beam 11 deflected in that manner, after leaving the lastdeflection magnet 7, scans a specific area surrounding the isocentre 10in concurrence with a predetermined treatment plan. Owing to thearrangement of the grid scanner 13, 14 between the last quadrupolemagnet 12 and the last deflection magnet 7, a high degree of flexibilitycan be achieved in the control, described in detail hereinafter, of thebeam magnitude and the beam dispersion at the isocentre 10.

The grid scanner magnets 13, 14 are controlled by a control device (notshown) that is a component of the overall supervisory control system ofthe ion beam therapy system.

There are provided, in the region between the beam outlet aperture ofthe last deflection magnet 7 and the isocentre 10, monitoring means formonitoring the treatment beam 11. Those monitoring means, which areprovided, for example, to ascertain and control the beam position, beamshape and particle flow, are explained in detail hereinafter.

As has already been mentioned hereinbefore, in addition a positronemitter tomograph (PET) can be provided for supervising the irradiationprocedure, the image recorder (camera) of which is aligned in an in-beamposition. The positron emitter tomography is preferably carried outduring the treatment or irradiation. When a treatment beam impinges ontissue, positron-emitting isotopes are generated from the primary ions.Some of those isotopes, which differ from the primary ions solely as aresult of the loss of one or two neutrons, stop almost in the sameregion as the corresponding primary ions. That stopping point of theso-called positron emitters can be determined for the purpose ofmonitoring the irradiation procedure by means of positron emittertomography.

There has been developed for the above-described ion beam therapy systeman extensive checking system, to be described in detail hereinbelow, forchecking and controlling the important performance features of thetherapy system.

A first section of this checking system is concerned with the generationof the treatment beam 11.

In addition to the checking, already described hereinbefore, of the typeof ion, at the same time the radiation energy of the treatment beam ismonitored. This is a requirement since it is necessary to adhere to theradiation energies required by the particular therapy. For that purpose,the monitoring means indicated in FIG. 2 comprise an absorber ionisationchamber system allocated to the isocentre 10 of the respective treatmentsite. The absorber ionisation chamber system measures the position ofthe Bragg peak at the treatment site for a few selected energy levels,which are activated during a therapy test cycle, the instantaneousradiation energy being derived from the measured position of the Braggpeak. In order to determine the position of the Bragg peak, the Braggcurves are measured in precise steps. If, on examination, there were tobe a departure of the Bragg peak from the desired position of more than0.5 mm, then intervention would be necessary. In order to examineconsistency, the described checking procedure can be carried out priorto each block of irradiation procedures.

A further point of detail with regard to checking of the treatment beamconcerns the monitoring of the level of intensity of the slowlyextracted treatment beam at the irradiation or treatment site. Thelimited dynamics of the grid scanner puts an upper limit on thedeflecting or scanning speed of the deflected treatment beam, thecomponent that determines that limitation being the maximumcurrent-increase speed of the magnet current supply devices. Thescanning speed of the treatment beam depends on the particular intensityof the beam and the planned particle coverage. In order to ensure thatthe maximum scanning speed is not achieved during the irradiation, theparticle rate extracted from the synchrotron 5 is not permittedsubstantially to exceed the desired value. If, on the other hand, therate falls distinctly short of that value, the total irradiation time isextended, the supervisory control and surveillance or monitoring systemin that case optionally being operated in the range of very small inputcurrents, which can adversely affect the accuracy of the beam detection.Accordingly, in the present therapy system, measurement and protocollingof the particle intensities in the synchrotron is provided in the upperintensity range and measurement and protocolling of the particle ratedelivered to the irradiation site is provided for all levels ofintensity for a plurality of energies over a few minutes. The particlerate fed from the accelerator to the irradiation site is between 2×10⁶and 2×10⁸ ions per extraction from the synchrotron 5. The departure ofthe particle rate from the predetermined desired value may be a maximumof 30% above and a maximum of 50% below that value. If those limitvalues are transgressed, an appropriate intervention is necessary. Inorder to check the consistency of the therapy system, such an inspectionmay, for example, be carried out daily.

The same dependencies of energy variation, intensity variation andfocusing variation must be taken as a basis for data supply for theaccelerator, for irradiation planning and for grid scan programming. Inorder to ensure that that is the case, the data inputs generatedaccelerator-wise after the last therapy programming should be comparedwith those used for the grid scan programming and irradiation planning.Departure from those data inputs is not permissible. In order to checkconsistency, such a check should be carried out prior to each block ofirradiation procedures.

During irradiation, the sections of the accelerator that are necessaryfor the therapy are blocked against (external) intervention in order toavoid intentional and unintentional false settings. At the same time,operational states are activated for all components and desired valuedata for the apparatus filed in the memories, e.g. EPROMS, exclusively,are accessed. The function of blocking the accelerator againstintervention can be checked by setting up a “super cycle” that containsboth test and therapy accelerators. Monitoring means or detectors, suchas, for example, (described in detail hereinafter) profile grids,luminous targets and ionisation chambers, are moved into the high energybeam guide 6 to the rotating cradle 8, and beam-influencing elements ofthe high energy beam guidance channel 6 and of the synchrotron 5 for thetherapy accelerator are deactivated. Blocking of the accelerator is thenactivated and all test accelerators are deactivated, while the therapyaccelerator is activated. In addition, all previously deactivatedcomponents are activated for the therapy accelerator, and the insertedprofile grids, luminous targets and ionisation chambers are moved outagain. Subsequently, switch-off commands are sent to individual magnetsand adjustment commands are sent to beam guidance diagnosis components,those commands normally not being allowed to have any effect owing tothe blocking of the accelerator. There is otherwise an error, which mustbe corrected accordingly. This check can be carried out prior to eachblock of irradiation procedures in order to check consistency.

It must be possible, for safety reasons, for the extraction of thetreatment beam from the synchrotron 5 to be terminated within less than1 ms after an appropriate signal from an interlock unit of the therapysystem. This is effected by a special quadrupole in the synchrotronrapidly being switched off. The time between a request by thesupervisory control and safety system for the beam to be terminated andthe absence of the beam at the irradiation site is of crucial importanceboth for the grid scanning operation when there is a change betweensuccessive isoenergy levels, those levels corresponding to areas to beirradiated with constant energy, and for a possible emergency shutdownof the system in case of error. There is accordingly provided a testthat measures the total time, that is to say both the reaction time ofthe request and the reaction time of the beam termination. To that end,the supervisory control system generates an appropriate signal whichsimulates the ending of an isoenergy level, or an interlock condition,that is to say a condition for an emergency shutdown, is generated. Theparticle count after a termination is then measured by the supervisorycontrol system, wherein 1 ms after termination the count is notpermitted to be greater than 10⁴ particles/s. In addition, using astorage oscillograph and a pulser, which are installed in fixed positionin the technical supervisory control room of the therapy system, ameasurement is carried out that evaluates the output signal of thecurrent/voltage converter of one of the ionisation chambers in order tocheck the afore-described measurement of the supervisory control system.In that second measurement, too, it should not be possible for any beamto be detected 1 ms after termination. The following time checks duringa termination should be made one after another: the beginning of theextraction time, the middle of the extraction time, the end of theextraction time and beyond the extraction time. The check should becarried out daily as a consistency check.

At the end of each irradiation procedure it is necessary, in respect ofthe accelerator, for a protocol to be drawn up that documents both thesettings of important accelerator components during the irradiationprocedure and selected beam diagnosis measurement results. In order totest the functionality of the protocolling and the protocol contents, itis proposed that a reference therapy cycle be activated and that theprotocol program be called up. The protocol data drawn up by theprotocol program can then be compared with the expected data,intervention being necessary when the protocol is incomplete or when aprotocolled apparatus error exists. In order to check consistency, thischecking procedure can be carried out prior to each block of irradiationprocedures.

A second section of the checking system is concerned with checking theguidance of the treatment beam (upstream of the irradiation site).

Starting from the accelerator, it must be ensured that termination ofextraction is effected when there is a termination request. Should thetreatment beam not be terminated by the termination request, that factis ascertained by the supervisory control system and safety system bymeans of an intensity measurement, and termination of the beam isrequested again by way of a separately provided redundant channel. Thatsecond request acts on a corresponding deflection dipole of the highenergy beam guidance channel 6. In order to check the functionality ofthat redundant termination of extraction, the alarm line provided forthe first termination of extraction is artificially interrupted. In thatcase, the afore-described second termination of extraction ought to betriggered automatically, which can be tested analogously to theabove-described test for the normal termination of extraction. Iftermination of extraction does not occur within 10 ms, appropriateintervention is necessary. In order to check consistency, that test canbe carried out prior to each block of irradiation procedures.

The operation of connection and disconnection of the dipoles arranged inthe high energy beam guidance channel 6 can be tested by means of afurther test. For reasons of patient safety, disconnection of the lasttwo deflection magnets in the high energy beam guidance channel 6 priorto irradiation (after blocking of the accelerator) is activatable onlyfrom the technical supervisory control room by way of special cableconnections to the power supply unit for those magnets. As a result ofsuch a disconnection, the beam supply to the irradiation site isstopped. Connection of those magnets can be carried out only from thetechnical supervisory control room by way of a special signal and cannot(as usual) be carried out from the main supervisory control room of theaccelerator. The operation of this connection and disconnection istested, the corresponding connections/terminals also being tested at thesame time. In order to check consistency, this test is carried out priorto each block of irradiation procedures.

A third section of the checking system is concerned with checking thebeam guidance at the irradiation site.

In accordance with a first aspect of that checking section, the zeroposition of the treatment beam is monitored. In order to ensure accuratepositioning of the beam at the isocentre 10 following deflection of thebeam 11 by the grid scanner magnets 13, 14, the axial position of thetreatment beam 11 in the last portion of the beam guide to theirradiation site must be checked for the entire energy and focusingrange. For that purpose, profile grids 16 are moved into the beam pathdownstream of the grid scanner magnets 13 and 14 and at the beam outletwindow, and test cycles are generated over the entire energy andfocusing range, in the course of which the profile grids are evaluatedindividually and the beam parameters ascertained in the procedure areprotocolled. When the profile grid arranged at the beam outlet window ismeasured, the profile grid 16 arranged upstream thereof must be movedout. By evaluating the beam parameters delivered by the profile grids itis possible to determine the beam position and the beam angle both inthe horizontal and in the vertical direction. From the beam positions ofthe profile grids, the position of the treatment beam to be expected atthe isocentre 10 is determined and then the protocol is checked. If aposition error of ±25% with respect to the required beam half-valuewidth is determined for the isocentre 10, appropriate intervention mustbe carried out. In order to check consistency, this test can be carriedout daily.

According to a further aspect of that checking section, the absolutebeam location and the location stability of the treatment beam at theirradiation site are checked. Adherence to the absolute beam position isa prerequisite for the implementation of the treatment or irradiationplans. The absolute location must therefore be measured usinglocation-sensitive detectors of the supervisory control system. Therelative location stability of the treatment beam in the isocentre ofthe irradiation site determines the accuracy with which an irradiationplan can be carried out. The location of the treatment beam is measuredand checked online, that is to say continuously, during an irradiationprocedure. If there are departures from the desired location inside atolerance limit predetermined by the irradiation plan, the irradiationis discontinued or an appropriate intervention is activated. Eachlocation-sensitive detector is checked separately.

The check is carried out using a profile grid and location-sensitivedetectors, such as, for example, multi-wire chambers.

When profile grids are used, the absolute beam position in the isocentre10 is checked by means of a luminous target or a film at the site of theisocentre. In that checking procedure, the position of the profile gridis adjusted with the isocentre made visible on the luminous target orfilm by a laser cross. By means of the grid scanner magnets 13, 14, thetreatment beam 11 is statically deflected into the isocentre 10 and thelocation coordinates obtained by the profile grid measurement arecompared with the predetermined desired values. This can be carried out,for example, at regular intervals, for example at approximately everytenth energy level.

When multi-wire chambers are used for the online examination and controlof the beam position, two multi-wire chambers are positioned at adistance of approximately 970 mm and 790 mm upstream of the isocentre 10and so aligned by means of a laser beam that the central beam extendingthrough the isocentre 10 runs perpendicularly through the centre of themulti-wire chambers. By means of the grid scanner magnets 13, 14, thebeam is statically deflected, for example at five different energies,into each of five different positions within the irradiation area (thatis, above and below in each case to the left and right, and also in thecentre). The location of the setting is measured by the supervisorycontrol system and compared with the desired values.

Since the multi-wire chambers are located at different distancesupstream of the isocentre, the projection of the irradiation field inthe two multi-wire chambers is reduced by different factors. By applyingthe rules of beam geometry and radiation law, the following reductionfactors are obtained:

Multi-wire chamber 970 mm upstream of the isocentre:

X coordinate: reduction factor 0.890

Y coordinate: reduction factor 0.876

Multi-wire chamber 790 mm upstream of the isocentre:

X coordinate: reduction factor 0.910

Y coordinate: reduction factor 0.899

Prior to the absolute beam position being checked by the multi-wirechambers, a calibration of their absolute positions should be carriedout. For that purpose, after alignment of and fixing of the position ofthe multi-wire chambers, a film positioned absolutely by means of theabove-mentioned laser cross is irradiated at five positions. The zeropoint of the beam determined by way of the film is compared with thatcalculated from the multi-wire chambers. The difference or discrepancythen gives correction offset values for calculating the location. Thosecorrection offset values are taken into consideration in the desiredposition values, the absolute position of all five points being comparedwith one another.

Using the multi-wire chambers calibrated in that manner, the absolutebeam position is then checked, control being so carried out that thedifference in position determined in that manner corresponds to amaximum of 25% of the half-value width of the beam profile. Thisintervention threshold relative to the half-value width of the beamprofile has proved practicable since all geometric parameters of anirradiation plan scale with the half-value width and, in particular, thequality of the generated particle coverages necessary for patientoperation is achieved. For carrying out a consistency check, only theafore-described multi-wire chamber measurements should be used, sincethe installation of an additional profile grid in the isocentre would bevery expensive for daily operation.

A further aspect of that checking section comprises the monitoring andcontrol of the absolute beam profile width and of stability over time.It is necessary to adhere to the beam focusing delivered by theaccelerator device according to the request by a pulse central controlof the supervisory control system, since the treatment or irradiationplans are based on those values. To that end, the absolute beam profilewidth in the isocentre 10 is checked with the aid of a profile grid, theposition of the profile grid being adjusted with the isocentre madevisible by a laser cross on a luminous target or on a film. Thetreatment beam is statically deflected by the grid scanner magnets 13,14 into the isocentre, it being possible for that to be carried out, forexample, at approximately every tenth energy level. The beam widthsobtained by the profile grid measurement are compared with predetermineddesired values, control being carried out in such a manner that amaximum departure of the beam width from the predetermined desired valueof ±50% is observed. This applies especially to the energy range above200 MeV/u.

Checking the consistency of the ion beam therapy system can on the otherhand be carried out using the multi-wire chambers, already describedhereinbefore, which are located, respectively, at a distance of 970 mmand 790 mm upstream of the isocentre 10. Before the actual checkingoperation, calibration of the absolute width measurement of the twomulti-wire chambers is carried out. In that procedure, a film isirradiated with horizontal and vertical stripes, each beam beinggenerated by an extraction from the synchrotron with fixed focusing. Inthat manner it is possible, depending on the selectable focusings, forexample for seven beams to be generated. The beam widths determined byway of the irradiated film are compared with those measured by themulti-wire chambers (location chambers) in order to obtain correctionoffset values therefrom that can then be taken into consideration againin the desired values. Then, by means of the thus calibrated multi-wirechambers in conjunction with the supervisory control system, thehalf-value width of the beam profile and its consistency or stabilityover time is measured and monitored, this being carried out especiallyat different energies and intensities for each of the selectablefocusings.

The above-described increase of the intervention threshold from 20% to50% of the half-value width in the measurement of the absolute beamprofile width compared with the measurement of the absolute beamlocation is compatible with the requirement for homogeneity, since thespacing of the beam positions in the context of the irradiation plan isset at 33% of the half-value width.

A few elements for analysis and modulation of the treatment beam areusually located upstream of the isocentre, such as, for example, thebeam outlet window, detectors or a ripple filter. Those elements bringabout a scattering of the treatment beam, which increases markedly asthe beam energy decreases. As a consequence, for physical reasons it isnot possible, or possible only with difficulty, to adhere to theoriginally requested beam width in the lower energy range (energies<200MeV/u). The result in that case would be that the upper tolerance valueswould be exceeded, and so that effect needs to be taken intoconsideration in irradiation planning.

The effects of the above-described monitoring and control measures withrespect to the beam zero position, the absolute beam location, theabsolute beam profile width and the stability thereof over time can beseen in FIGS. 3A/B and FIGS. 4A/B. FIGS. 3A and 3B correspond to anenlarged view of the diagrams shown in FIGS. 4A and 4B, respectively,FIGS. 3A/4A showing beam positions without the position control proposedhereinbefore and FIGS. 3B/4B showing beam position with the positioncontrol. It can be seen from the diagrams that, as a result of using theposition control etc., a considerably more stable beam position can beachieved, whereas without position control there are some markeddepartures from the desired beam position.

A further aspect of this checking section is concerned with monitoringthe particle count in the treatment beam, that is to say, monitoring thevariation in the particle count. So as to prevent the measurement rangefor particle count measurements from becoming too large, the intensityof the treatment beam delivered by the accelerator should vary onlywithin certain tolerance limits. It is proposed in the present case thatthe intensity of the treatment beam be measured using the ionisationchambers in conjunction with the measurement apparatus of thesupervisory control system, and that the particle count be averaged overa time window of 300 μs. The particle counts then measured are permittedwithin the time window to correspond to a maximum of five times thevalue of the previously ascertained average value in order not totrigger an intervention. As a result of taking those steps, a morereliable measurement range can be selected with which even particlecounts that are, for example, higher by a factor of 10 than thepreviously calculated average value can still be measured correctly.Should even higher particle counts arise, an alarm is triggered and theinterlock unit, already mentioned, triggers switching off of the beam.Care must be taken, however, that this checking aspect relates only tothe presetting of the detectors, and has no direct influence on theenergy dose or the like. Even in the event of a variation in theparticle count lying appreciably above the previously definedintervention threshold, the homogeneity of the particle coverages,described hereinafter, may be sufficient as a decisive qualitycriterion.

Finally, with regard to a reliable and stable beam guidance at theirradiation site, the desired positions of all movable componentsbetween the last deflection magnets of the high energy beam guidancechannel 6 and the rotating cradle 8 should be checked regularly, sinceany object located in the beam guide has an adverse effect on the beamquality at the irradiation site. It must therefore be ensured that nomovable components of the beam guide are to be found in the beam path.To that end, there are connected to the corresponding movable componentslimit switches of which the states can be automatically and individuallychecked by the supervisory control system. In order to checkconsistency, this should be repeated prior to each block of irradiationprocedures.

A fourth section of the checking system is concerned with checkingfeatures that are associated with the irradiation control unit of theion beam therapy system.

The electric charge generated in the afore-described ionisation chambersof the supervising or monitoring system of the therapy system, whichcharge serves to determine the particle count, depends on the pressureand the temperature of the ionisation chamber gas, so that both thosevariables have to be monitored and protocolled during irradiation. Thepressure and the temperature of the gas of the ionisation chambers aremeasured by means of electrical sensors, the measurement values beingascertained approximately once per minute by the supervisory controlsystem and converted with entered calibration factors into absoluteunits (hPa and ° C.) and displayed digitally. The trend of themeasurement values over time can be illustrated graphically in a trenddiagram. The sensors are calibrated by means of reference measurementdevices. The calibration of the sensors installed in the ionisationchambers should be repeated prior to each block of therapy irradiationprocedures. In addition, the atmospheric pressure and the roomtemperature at the site of the monitoring system are measured byabsolutely calibrated devices and ascertained by the supervisory controlsystem and also protocolled in each irradiation procedure. Consequently,for the (daily) checking of the ionisation chambers, the absolute valuesfor atmospheric pressure and room temperature can be read off directlyat the reference measurement devices, compared with the values displayedby the supervisory control system and protocolled. The measurementvalues registered in the daily calibration of the monitoring systemserve as reference values in that procedure. If there is a discrepancyof 20 hPa or 5° C., an alarm is triggered by the supervisory controlsystem.

In addition, the loading of programs and data sets into the controlcomputers of the ion beam therapy system must be checked. This isnecessary in order to be able correctly to load data required for theirradiation of a patient into the sequence control of the system. Onlyif all data are correct may irradiation of a patient be commenced. Forthat purpose, using special programs in the server computers of thesupervisory control system, programs and data are written into theindividual processors of the control computers, read back and comparedwith the programs and data stored in the individual memories, suchchecking programs being performed automatically prior to eachirradiation procedure. Only when the reloaded data correspond preciselyto the data stored in the data memories of the supervisory controlsystem is it possible to start from a state of being safely undercontrol. When discrepancies exist, an alarm signal is generated and theafore-described interlock unit, which serves to prevent an irradiationprocedure, cannot be released.

A further checking aspect is concerned with the connection of thecurrents for the deflection magnets 13, 14 of the grid scanner. Caremust be taken that the current values of those deflection magnetsachieve a specific desired value set in the magnet supply devices, bothin terms of value and time, within specific tolerance limits. For thatpurpose, the time between setting a magnet current value in the magnetsupply devices and reaching the appropriate stable magnet current ismeasured for different current values. The maximum current accuracy thatcan be tolerated in respect of a departure from the set magnet currentvalue is 0.3 A. The maximum adjustment time that can be tolerated whenthere is a current change of 2 A is 175 μs in the x direction and 325 μsin the y direction. When those tolerances are not adhered to, theirradiation must be terminated. In order to check consistency, this testcan be carried out prior to each block of irradiation procedures.

Finally, it must also be ensured that the number of irradiation pointsactive when a termination condition arises is stored permanently, thatis to say, safeguarded against loss of power. This renders possiblecontinuation at a later point in time of the irradiation approved byauthorised personnel. The functionality of this implemented safetyfunction can be checked by loading a particular irradiation or treatmentplan into the supervisory control system and carrying out the planwithout irradiation, that is to say simulating it. At a particularirradiation site, the voltage supply of the sequence control is switchedoff and, after restarting the system, the last irradiation site is readout and compared with the irradiation site when the voltage supply wasswitched off. If the two readings do not agree, appropriate interventionis carried out. To check consistency, this inspection is carried outprior to each block of irradiation procedures.

A fifth section of the checking system is concerned with checking thefunctionality of the interlock unit, already described hereinbefore, ofthe ion beam therapy system.

Accordingly, for example all apparatus parameters relevant from a safetystandpoint for triggering an emergency shutdown of the system when aninterlock event or interlock condition exists must be checked. Shutdownof the treatment beam 11 can be carried out only when an interlock eventis detected. Therefore all sources that can lead to an interlock eventmust be individually simulated in a test and the triggering of theinterlock, that is to say the generation by the interlock unit ofsignals that result in the emergency shutdown of the treatment beam 11,must be checked. During operation, the interlock unit monitors, forexample, the signals of the above-described limit switches of themovable components in the beam guide, the states of the magnet supplydevices of the grid scanner magnets 13 and 14, the ionisation chambersin respect of the voltage supply, a data overflow of the data transfer,the adherence to the intensity limit values and the synchronization ofthe individual ionisation chambers, the electronics of the beam positionmeasurement device and the beam position itself, the high voltage andthe gas flow of the individual detectors, a possible interlock by thesequence control computer, the position of the patient couch, a possibleinterruption of the immobilisation of the patient (for example when themask at the irradiation site is opened or when the patient moves), thereadiness for operation of all computer programs and a possibleemergency shutdown or release of an irradiation procedure by the medicaloperating console of the therapy system etc. If triggering of theinterlock does not occur when an interlock condition exists,intervention in the therapy system and elimination of the errors isnecessary. To check consistency, this inspection should be carried outdaily.

The functionality of the manual emergency shutdown by way of the medicaloperating console must likewise be checked, since manual emergencyshutdown must be guaranteed at all times.

Finally, it is necessary to check on the individual consoles of the ionbeam therapy system, especially of the technical supervisory controlrooms and of the main supervisory control room, the displays of all ofthe conditions that are relevant in terms of safety. The display ofthose safety-relevant conditions serves for the rapid detection andelimination of errors and gives the operating personnel informationconcerning the current state of the irradiation procedure. Thosedisplays of the alarm conditions can be checked together with theabove-described test of the interlock unit. In order to checkconsistency, this test should be carried out prior to each block ofirradiation procedures and after each change of the supervisory controlsystem or of the programs.

A sixth section of the checking system is concerned with checking of themedical devices for the patient positioning of the ion beam therapysystem.

Thus, for example, the accuracy of the stereotactic determination ofcoordinates of a target point should be checked by means of a CT or MRprocedure, since the accuracy of the stereotactic image formation is acrucial factor for the overall accuracy of the irradiation. For thatpurpose, it is possible for any desired target point to be representedinside a spherical phantom by means of a special specimen body, thecentre point of which can be visibly represented by means of theimage-forming method. The spherical phantom is inserted into thestereotactic frame so that the centre point becomes an unknown targetpoint. The stereotactic coordinates are then ascertained one after theother in terms of time using the applied X-ray, CT or MR method, whereinin the tomographic method the layer spacing should be 1 mm. Since theX-ray method is accurate to {fraction (1/10)} mm, the accuracy of thedetermination of the target point by CT and MR can be ascertained bycomparison with the X-ray method, that is to say, the radial spacingbetween the position of the target point determined by X-ray image andthe position determined by the CT or MR method is checked. The radialspacing should not exceed 1.5 mm. For the purpose of checkingconsistency, it is sufficient for this test to be carried out annually.

As a further checking aspect it is proposed that the accuracy of theposition of the isocentre between the axis of rotation of the patientcouch 9 and the central beam 11 of the grid scanner 13, 14 be checkedsince the isocentre, defined as the point of intersection between theaxis of rotation of the patient couch 9 and the central beam 11 of thegrid scanner 13, 14, is the connecting element in the positioningbetween planning and irradiation. A check for consistency should becarried out prior to each block of irradiation procedures.

In order to check the isocentre in relation to the axis of rotation ofthe patient couch 9, a metallic specimen body (2-3 mm in diameter) isintroduced, with the aid of lasers, into the nominal isocentre, that isto say into the nominal axis of rotation of the patient couch 9. Thespecimen body is maintained in fixed position by means of a plumb bob,which is centred precisely on the centre point above the specimen body.On rotation of the patient couch 9 about the axis of rotation, theextent to which the specimen body moves in relation to the plumb bob isascertained. This procedure is carried out at at least three differentlevels of the patient couch 9, with a maximum displaceability of thepatient couch 9 up or down of 15 cm, for example at the level of theisocentre 10 and with a minimum of 15 cm distance above and below. Themaximum departure allowable is 1.0 mm in the direction of the beam andonly 0.5 mm perpendicular to the direction of the beam. Variations thatare in the beam direction are less critical, since dose distributions inthe patient are not affected by such variations.

In order to check the isocentre in relation to the central beam 11, theposition of the isocentre is, by definition, fixed on the axis ofrotation of the patient couch 9 below the plane for the straight-aheadbeam, and is ascertained relative to wall markers by means of an opticalmeasurement system. Checking the position of the specimen body relativeto the central beam 11 is carried out by a film measurement, averification film being irradiated, downstream of the specimen bodyviewed in the direction of the beam, with a (undeflected) central beam,the half-value width of which is greater than the diameter of thespecimen body, with the result that the position of the specimen body isprojected on the verification film relative to the central beam. In thiscase the intervention threshold is at a maximum 25% departure from thehalf-value width of the primary beam.

In addition, the accuracy of the laser alignment on the isocentre 10must be checked, since the lasers mark out the isocentre 10. In thisprocedure, following positioning of the specimen body in the isocentre10, the lasers are aligned onto the centre point of the specimen body bymeans of optical measurement, and the departure of the laser lines fromthe horizontal and vertical are checked, the maximum departure allowedin each case being 1 mm. In order to check consistency, the image of thelasers on the opposite-lying walls or on the floor is marked out andthen used as a reference value.

A further checking aspect is concerned with the accuracy of thealignment of the X-ray tubes and of the target cross on theopposite-lying recording stations, since the X-ray method represents anadditional procedure for marking out the isocentre 10. After positioningthe specimen body in the isocentre 10 by means of optical measurement,that is to say using lasers, X-ray images are taken in the three spatialdirections and the spacing between the projected image of the specimenbody and the target cross on the X-ray image is ascertained. The imageof the specimen body should be projected precisely onto the image of thetarget cross, so that the maximum permissible spacing between theprojected image of the specimen body and the target cross is 1 mm.

Owing to the isocentric irradiation of the patients, it is alsonecessary for the accuracy of the display of the angular scale of theisocentric rotation of the patient couch 9 to be checked, and this canbe carried out analogously to the provisions of DIN 6847-5, point12.2.4. The maximum tolerable inaccuracy is 1°.

The spatial stability of the isocentric rotation of the patient couch 9should likewise be checked, since a corresponding stability is aprerequisite of the definition of the isocentre 10. This check can becarried out analogously to DIN 6847-5, point 14.2, the interventionthreshold being an inaccuracy of 1 mm.

It is finally also proposed that the accuracy of the placing andpositioning of the patient is checked, since accurate patientpositioning is a prerequisite for proper irradiation for the tumour inquestion. In that connection, for the inspection test and to checkconsistency (prior to each block of irradiation procedures) of thetherapy system, the unknown stereotactic coordinates of the centre pointof a specimen body, which has been fixed within the stereotactic basering, are ascertained as the target point and, with the aid of thestereotactic targeting device and by means of transverse movement of thepatient couch 9, the centre point is brought into the isocentre 10. Inthat position, X-ray images are taken in the three spatial directionsand the spacing of the position of the specimen body from the targetcross is determined on the three images. The maximum radial spacingallowed between the centre point of the specimen body and the isocentreis 1.5 mm. Otherwise an appropriate correction of the placing of thepatient is necessary.

A seventh aspect of the checking system is concerned with irradiationplanning, in the course of which especially the radiation dose valuesintended for a particular irradiation procedure are calculated.

First of all it must be ensured that it is always the same ground datasets that are used for planning irradiation procedures, that is to sayfor calculating each radiation dose. This can be effected by comparingthe name, the date and the size of the data files containing the grounddata with the correct designations of a previously taken backup copy.This happens automatically each time the dose calculation algorithm iscalled up.

Also, the identity of the values of the actual ground data sets with thecorresponding values of a backup copy must be checked in order to ensurethat the ground data sets have not been changed in an uncontrolledmanner. Also carried out here is a comparison of the contents of theactual ground data sets with the backup copy by means of a computerprogram, which program should be initiated especially prior to eachblock of irradiation procedures.

According to DIN 6873 part 5, irradiation planning systems, it isnecessary in addition for the reference values in the ground data set tobe checked once a month. This point of detail can be omitted in thepresent irradiation planning with heavy ions since the depth dosedistributions, that is to say the energy loss data as a function of thedepth, are stored as absolute values relative to the input fluence. Nospecial reference value for the dose is therefore recorded. The grounddata sets used are already checked in the manner described above.

An important aspect in checking the irradiation planning is checking theaccuracy of the dose calculation (carried out automatically in the ionbeam therapy system) for a planned irradiation procedure as a functionof the ground data present and the dose calculation algorithms used, itbeing necessary to distinguish between the irradiation of a homogeneousand a non-homogenous medium. In both cases checking the dose calculationcan be carried out by using a phantom, and this procedure is describedin detail below.

In order to check the calculated dose for a homogeneous medium in theirradiation planning program of the ion beam therapy system, a pluralityof measurement points, for example 10 measurement points, are defined inthe calculated dose distributions or CT sections, at which measurementpoints the calculated physical dose is to be verified experimentally.The verification is carried out in a water phantom, ionisation chambersbeing positioned in the water phantom at the coordinates correspondingto the desired measurement points. The irradiation planning programcalculates for the individual measurement points, in addition to theenergy dose values related to water, also the coordinates thereof in thephantom used. The phantom is then irradiated using the controlparameters calculated by the irradiation planning program, the valuesascertained by the ionisation chambers being converted into energy dosevalues in order to verify the calculated dose values.

The verification is carried out for a plurality of irradiation plans,preference being given to the verification of six typical irradiationplans of which three are concerned with hypothetical target volumes inthe water phantom and three are concerned with the irradiation ofpatients. The latter irradiation plans are thereafter used as standardpatient plans. The values calculated by the irradiation calculationprogram serve as reference values for the consistency check that is tobe carried out.

The intervention threshold laid down is that the maximum permitteddiscrepancy between the calculated and the measured radiation dosevalues is in total, that is to say on average, ±5% of the dose of thetarget irradiation volume. It is in addition laid down that the maximumpermitted discrepancy for an individual measurement point is ±7%.

The above-described procedure relates especially to the inspection testof the ion beam therapy system. In order to check consistency, it issufficient to verify only two in each case of the above-describedstandard plans to check the consistency of the calculated dosedistributions, and to compare those with the dose distributions to bedetermined experimentally. The consistency check should be carried outprior to each block of irradiation procedures.

In order to check the accuracy of the dose calculations as a function ofthe ground data, of the irradiation calculation algorithms used and ofthe approximation for a non-homogeneous medium used, a sphericalsolid-body phantom can be employed that consists of a materialequivalent to water and is constructed from individual layers into whichvarious non-homogeneities can be inserted in order to simulate differentnon-homogeneous bodies. Those non-homogeneities are disks, which consistof various tissue-equivalent materials (for example corresponding to thematerial of the lungs, of a soft or hard bone, of soft parts or of boundwater) or simply air (when a disk is not inserted). In that case, too,up to 10 measurement points are defined in the phantom for theverification, at each of which the radiation dose is both calculated bythe irradiation planning program and ascertained using a group ofsimultaneously measuring ionisation chambers and compared therewith.

It is proposed, for the inspection test, that three different phantomstructures be made for investigating the calculated dose distributionbehind boundary layers of different materials (for example air/water andbone/water), in thin non-homogeneities and in thick non-homogeneities.

The proposed tolerance threshold in the investigation of the calculateddose values for non-homogeneous media is a maximum permitted averagediscrepancy between the calculated dose values and the measured dosevalues of all measurement points of ±5% and a maximum permitteddiscrepancy for an individual measurement point of ±7%. In order tocheck consistency, the above-described tests can be carried out prior toeach block of irradiation procedures.

The dose calculations can also be verified by using an irregularlyshaped test phantom. In that case use is made of a test phantom thatconsists of water-equivalent material and, for example, is modelled on ahuman head. As described hereinbefore, up to 10 measurement points aredefined in the phantom for the verification. Furthermore, irradiationparameters are laid down for a suitable target irradiation volume in thehead phantom and the test phantom is aligned with the aid of thestereotactic base ring. The energy dose values related to watercalculated by the irradiation planning program of the ion therapy systemat the selected measurement points are then compared with reference tothe values measured at those measurement points with the ionisationchambers, and again the maximum permitted discrepancy for allmeasurement points is ±5% of the dose of the target irradiation volume,whereas the maximum permitted discrepancy for each individualmeasurement point is ±7%. In order to check consistency, this test canbe carried out prior to each block of irradiation procedures.

A further aspect in checking the irradiation planning concerns theinspection of the image-forming process used in the ion beam therapysystem so as to ensure a correct transfer of the geometric structures(for example of the target irradiation volume and of the contours of thepatient) and of the planning parameters from the image formation to thepositioning. For that purpose, as in the verification of the calculatedradiation dose values in non-homogeneous medium, a phantom havingdisk-shaped or ring-shaped inserts can be used, it being possible forthe non-homogeneous inserts in this case in addition to have differentdiameters. An image of the phantom is taken, and from the CT data soobtained digital X-ray reconstructions are calculated for the three maindirections in the rotating cradle 8 (see FIG. 2). A verification of theplanning geometry is then carried out in the three main directions withthe aid of X-ray images of the X-ray positioning system. That procedurecan be carried out at different angles of the patient couch 9 shown inFIG. 2, for example at 0°, 45° and 90°. In that manner the shape and theposition of the non-homogeneities in the digital X-ray reconstructionrelative to the X-ray images of the X-ray positioning system areverified. The tolerance thresholds laid down in this case are that boththe maximum permitted positional discrepancy and the maximum permitteddiscrepancy in respect of the shape of the rings of the phantom is 2 mm.The consistency check can again be carried out prior to each block ofirradiation procedures.

In order to increase operational safety, it is in addition necessary tomonitor the maintenance and further development of the irradiationplanning programs used in the ion beam therapy system. It is possible,following a further development of the irradiation planning programs,that an incorrect version of the program may be used in error. In orderto avoid that and to ensure that the correct versions of the variousmodules are always used, the supervisory control system of the ion beamtherapy system is so constructed that, each time an irradiation planningprogram is called up, version numbers with the date of the respectiveprogram are displayed which are to be compared by the user with data ina protocol book.

It must likewise be ensured, in the event of a further development ofthe irradiation planning program, that is to say when a new versionexists, that that version becomes effective only after a renewedinspection test. This can be effected by complete dose distributionsbeing calculated as described hereinbefore for a homogeneous medium, anon-homogeneous medium and for an irregularly shaped phantom, and beingstored as a backup copy. When the new program version is used, thosestored dose values can be used as reference values for verification ofthe functionality of the new program version, since it will also benecessary when the new program version is used for the same dose valuesto be calculated for the same phantom. This check should therefore becarried out after any change to the irradiation planning program.

An eighth section of the checking system is concerned with theinspection of the grid scanning procedure and the dosimetry.

A first checking aspect of this checking section is concerned with theparticle count monitoring or supervising means of the ion beam therapysystem, which in the case of the present exemplary embodiment—as hasalready been described—consist of large-area ionisation chambers.

In that connection, for example the consistency of the calibrationfactors of those ionisation chambers must be checked, since thecalibration factors are permitted to vary only within the bounds ofvariations in atmospheric density. The two ionisation chambers of thegrid scanner are calibrated in respect of the particle count persupervising or monitoring unit of the ionisation chambers. Thecalibration is described by a calibration factor K, which depends on theirradiation energy E of the particles and the step width Ax and Ay ofthe grid scanner, that is to say, K=K(E, Δx, y). The calibration of theionisation chambers is carried out by a dose measurement in ahomogeneously deflected irradiation field, wherein the discrepanciesfrom the reference conditions are corrected and the display of theionisation chamber is converted into an energy dose related to waterD_(scan). The calibration factor is calculated according to:

K(E, Δx, Δy)=(D _(scan) /M _(i)).Δx.Δy/(S(E)/ρ)

in which (S(E)/ρ)=mass stopping power of ¹²C at an irradiation energy E,and

M=monitoring units per coordinate point i of the ionisation chamber.

The relevant energy range (for example from 80 MeV/u to 430 MeV/u) ismeasured in a plurality of steps. The measurement site of the particularionisation chamber checked is located in the isocentre 10, theionisation chamber or the dosimeter being arranged in a solid-bodyphantom. The same table of the mass stopping power of ¹²C is used asthat on which the irradiation planning is based. In that manner,depending on the energy E and the step width Δx,Δy, a group ofcalibration factors K is obtained, the maximum permitted discrepancyfrom the reference values for each calibration factor being ±3%. Fromthe group of calibration factors at least three values should bechecked. In order to check consistency, this test procedure should becarried out daily.

The dose consistency must also be checked, since identical preselectedmonitoring units of the ionisation chambers must always result inidentical dose displays. It is therefore recommended that theconsistency of the dose be checked in the centre point of cubiformirradiation volumes, which are generated or scanned by the grid scanneror the magnets 13, 14 thereof, as a function of the group of calibrationfactors of the ionisation chambers. For that purpose, in order to obtainreference values the dose is measured in a phantom that is so positionedthat the isocentre 10 is located precisely in the centre of its frontface. In that arrangement, the irradiation is carried out inside anirradiation cube or dose cube with an edge of 5 cm length, the centre ofwhich is arranged as measurement site at 11.3 cm water-equivalent depth.(The calculation of the control data for the generation of the dose cubeis carried out by means of CT-based irradiation planning. For that stepit is more advantageous to arrange the isocentre 10 on the site at whichthe beam enters the water phantom. Furthermore, the selected measurementdepth renders possible standardisation of the measurement equipment forthe different tests). The radiation dose determined in that manner isstored as a reference dose. The actual dose values measured subsequentlycan then be compared with that reference dose, the maximum permitteddiscrepancy between the actual and the nominal dose (reference dose)being ±3%. A daily consistency check should be carried out.

Also, the parameters influencing the particle count monitors andionisation chambers need to be checked, there being checked in thatprocedure especially the dependency of the calibration factors K on theparticle fluence and the particle flow. In both cases an annualconsistency check should be carried out.

In order to check the dependency of the calibration factors on theparticle fluence, the procedure carried out is essentially the same asfor checking the consistency of the calibration factors. Themeasurements are carried out in a phantom, which with an area of 5×5 cm²is irradiated at energies of 150 MeV/u, 250 MeV/u and 350 MeV/u with thesame beam intensity in each case. An ionisation chamber is arranged inthe centre of the irradiated surface area. The monitoring values of theionisation chamber are so laid down that a dose of 0.2 Gy, 0.5 Gy and 1Gy, respectively, is produced at the measurement site. For thosedifferent monitoring values, concordance between the actual and thenominal dose is ascertained, a maximum discrepancy of ±3% beingpermitted. Adherence to that narrow tolerance is expedient and alsopracticable.

To check the dependency of the calibration factors on the particle flow,the procedure used is likewise substantially the same as that used tocheck the consistency of the calibration factors. In this case, however,the dose is kept constant and the beam intensity is in each case set ata high, a medium and a low value, so that concurrence of the actualradiation dose with the nominal reference dose can be checked fordifferent intensities. In this case, too, a maximum discrepancy of ±3%is permissible.

Regarding the ionisation chambers and particle count monitors, thedependency of the calibration factors thereof on the beam positionshould be checked. Substantially the same procedure as that used tocheck the consistency of the calibration factors is carried out, but thearrangement employed is the same as that used in the above-describedchecking of the dose consistency. The measurements are carried out in anirradiation volume or irradiation cube of the grid scanner 13, 14 withan edge of 5 cm length, but with a lateral displacement of 2 cm and 6cm. The monitoring values of the ionisation chambers are laid down suchthat a radiation dose of 1 Gy is produced in the centre of theirradiation volume. In checking the displays of the ionisation chambers,the value measured at the side should differ by no more than 3% from thevalue measured in the centre. In that case, too, an annual consistencycheck is recommended.

A further checking aspect of this checking section is concerned with theinspection of the dose distribution of the grid scanner 13, 14, in whichboth the depth distribution of the dose and the transverse distributionof the dose are examined.

The homogeneity of the depth distribution of the dose is checked as afunction of a selected irradiation energy and selected monitoring valuesper irradiation energy value of the ionisation chambers used, since thedepth dose homogeneity is crucially dependent on the energy selected andthe consistency thereof. For that purpose, again parallelepipedal orcubic irradiation volumes are generated in a phantom with the gridscanner magnets 13, 14, wherein for each coordinate point of a layer(energy) a constant particle coverage, but a different particle coverageper layer, is used in such a manner that a homogeneous dose distributionis achieved in the irradiation cube. A plurality of dosimeters(ionisation chambers), for example 10 ionisation chambers, carry outmeasurements in different water-equivalent depths, the ionisationchambers being so positioned that irradiation of a plurality ofionisation chambers one behind another does not occur. The edge lengthsof the irradiation cubes are, for example, 2.5 cm, 5 cm and 10 cm, themeasurements of the ionisation chambers being carried out for depths ofthe centre points of the respective cube-shaped irradiation volume of 5cm, 12.5 cm and 20 cm respectively. The monitoring values areestablished from the irradiation planning by a radiation dose,predetermined by the irradiation planning, being produced in the centreof the respective irradiation volume. By comparison of the actualmeasurement values with the reference values, the degree of variation ofthe displays of the ionisation chambers can be checked. A maximumdiscrepancy of ±5% can be tolerated. If that tolerance limit isexceeded, there must be a system intervention in order to correct theexcessively large discrepancy. In order to check consistency, theabove-described checking procedure should be carried out prior to eachblock of irradiation procedures.

The transverse distribution of the dose of the grid scanner is checkedas a function of the energy in order to ensure that the homogeneity ofthe grid scan procedure is guaranteed at all irradiation energies used.In that case, when the ionisation chamber monitoring values are fixedand in each case irradiation energies are different (for example 100MeV/u, 150 MeV/u, 200 MeV/u, 250 MeV/u, 300 MeV/u and 350 MeV/u) andbeam fields are different, the radiation dose perpendicular to thedirection of the beam is ascertained with a plurality of ionisationchambers measuring simultaneously. At the same time, open air in frontof the dosimeters or ionisation chambers produces a blackeningdistribution on a verification film. With the grid scanner 13, 14, areaswith a lateral face of, for example, 5 cm, 10 cm and 18 cm are produced,wherein the radiation dose should in each case be approximately 1 Gy.The standard deviation of the corrected displays of the ionisationchambers or of the verification film blackening inside the irradiationfield is checked, the maximum tolerable departure from the referencevalues being ±5%. Non-tolerable departures from the reference values arecorrected in order for adaptation to the measurement conditions thatactually exist to be achieved. A consistency check should be carried outprior to each block of irradiation procedures, the use of theverification film with monitoring of the blackening of that verificationfilm being sufficient in this case.

A further checking aspect of this checking section is concerned with theinspection of the field geometry in the grid scan procedure, thedependency of the spatial position of a particular irradiation volume ofthe grid scanner 13, 14 on selected irradiation energies being checked.For that purpose, cubic or parallelepipedal irradiation volumes aregenerated by the grid scanner 13, 14, wherein a constant particlecoverage is used for each coordinate point of a layer (energy), but adifferent coverage is used per layer, such that a homogeneous dosedistribution is obtained in the irradiation cube. Under those conditionsa wedge-shaped solid-body phantom is irradiated, behind which averification film is located. The position of the verification filmblackening relative to the centre point of the irradiation is thendetermined.

In the measurement, the edge lengths of the irradiation fields are, forexample, 4 cm , 7 cm and 12 cm, while the extent of the irradiationparallelepipeds or cubes in the direction of the beam is 2.5 cm, 5 cmand 10 cm. The measurements are carried out for water-equivalent depthsof each of the centre points of the irradiation volumes of 5 cm, 12.5 cmand 20 cm respectively. The monitoring values of the dosimeters orionisation chambers are so determined from the irradiation planning thata radiation dose predetermined by the irradiation planning is producedin the centre of the irradiation volume. There are defined as the fieldboundaries the locations at which the marginal fall-off in theblackening is 50% of the plateau value. The position of the distal fieldboundaries and of the lateral field boundaries viewed in the directionof the beam are examined and compared with reference values. A departureof 2 mm in each direction is tolerable, otherwise a correction of thesystem must be carried out in order to adapt the system to themeasurement conditions actually existing. In order to check consistency,that checking procedure should be carried our prior to each block ofirradiation procedures, a selection of in each case three conditionsfrom the combinations of the above-described conditions being sufficienthere.

Finally, a further checking aspect of this checking section is concernedwith the verification of the overall system so as to be able to verifythe accuracy of the applied radiation dose, in terms of its level andspatial dimensions, for each of the patients to be irradiated, so that acorrect cooperation of the individual components of the system isensured. In this procedure it is necessary to distinguish between theirradiation of a homogeneous medium and the irradiation of anon-homogeneous medium.

In the first case, as in the above-described verification of theconcordance of calculated and measured dose distributions for ahomogeneous medium, a homogenous phantom is used and essentially thesame procedure is carried out, but with the exception that in this caseindividual patient irradiation plans are used as a basis. For allmeasurement points, the difference between the calculated radiation doseand the measured radiation dose is ascertained, and again an averagediscrepancy for all measurement points of 5% and a discrepancy for anindividual measurement point of 7% is tolerable. In order to checkconsistency, this test should be carried out prior to each block ofirradiation procedures.

In order to check the accuracy in the case of irradiation of anon-homogeneous medium that is to be irradiated, again a non-homogeneousphantom is used, wherein in this case irradiation planning is carriedout by preparing only one hemispherical phantom from a solid,water-equivalent material having a radius of, for example, 8 cm. Forirradiation planning, the centre point of the phantom is located in theisocentre 10 and the hemisphere of the phantom is opposite to thedirection of irradiation. Various non-homogeneities, for example in theform of disks each having a diameter of 3 cm, can be inserted into thephantom, preference being given to the use of seven different materialsor non-homogeneities having the following densities:

No. Density

1 0.001 (air)

2 0.30 (lungs)

3 1.035 (bound water)

4 0.92 (fat)

5 1.05 (muscle)

6 1.14 (soft bone)

7 1.84 (hard bone)

The planned target irradiation volume is, for three different directionsof irradiation having an angle of irradiation of 0°, +45° and −45°, ineach case a 2 cm-thick layer inside the hemispherical phantom, whichdirectly adjoins the flat face of the hemisphere, so that the distalposition of the irradiation volume coincides with the rear flat face.The homogeneous radiation dose planned in the target irradiation volumeis 1 Gy. With those control data for controlling the grid scanner, theirradiation procedures are carried out with the three directions ofirradiation, there being positioned both in the target irradiationvolume and behind each non-homogeneity a dosimeter (that is, anionisation chamber), the display of which is monitored. The ascertainedenergy dose at all of the measurement points inside the targetirradiation volume should not exceed the threshold 1 Gy ±5%, whilst 5 cmbehind the target irradiation volume the maximum tolerable departurefrom the calculated radiation dose relative to the target irradiationvolume is ±10%. In addition, for all measurement points again an averagediscrepancy of the measured radiation dose of ±5% is tolerable, and foreach individual measurement point a maximum discrepancy of ±7% istolerable. In order to check consistency, this checking procedure shouldbe carried out prior to each block of irradiation procedures.

What is claimed is:
 1. Method of operating a heavy ion beam therapysystem with monitoring of the radiation dose distribution, wherein theheavy ion beam therapy system comprises at least one ion source (1) anaccelerator device (2, 5) for the acceleration of the ions of the ionsource (1) in the form of a heavy ion beam (11), a beam guidance system(6, 8), to guide the heavy ion beam (11) from the accelerator device (2,5) to at least one irradiation site for treatment of a patient, the beamguidance system (6, 8) comprising at least one beam guidance channel(6), and a grid scanner device, arranged in the beam guidance system (6,8), having vertical deflection means (13) and horizontal deflectionmeans (14) for the vertical and horizontal deflection of the heavy ionbeam (11) perpendicular to its beam direction, with the result that theheavy ion beam (11) is deflected by the grid scanner device to anisocentre (10) of the irradiation site and scans a specific areasurrounding the isocentre (10) using a specific radiation dose,characterised in that the radiation dose distribution of the gridscanner device (13, 14) at various positions in the region of theisocentre (10) is measured and evaluated; and it is concluded that theradiation dose distribution is adequately homogeneous if the degree ofvariation in the radiation dose values measured at the individualpositions does not exceed a specific tolerance limit value; andmonitoring means are calibrated in accordance with a calibration factorK in respect of the particle count, ascertained by those means, permonitoring unit of the monitoring means, the calibration factor K beingcalculated, as a function of the irradiation energy E of the particlesof the heavy ion beam (11) and the scanning step width Ax and Ay of thegrid scanner device (13, 14), by means of: K(E, Δx, Δy)=(D _(scan) /M_(i)).Δx.Δy/(S(E)/ρ) in which the expression (S(E)/ρ) corresponds to themass stopping power of the particles of the heavy ion beam (11) at anirradiation energy E, M corresponding to the number of monitoring unitsper coordinate point i of the monitoring means, and D_(scan)corresponding to the radiation dose value ascertained by the monitoringmeans and converted into an energy dose value related to water. 2.Method according to claim 1, characterised in that the radiation dosedistribution of the grid scanner device (13, 14) in the beam directionof the heavy ion beam (11) is measured and evaluated.
 3. Methodaccording to claim 2, characterised in that irradiation of a phantom iscarried out using the grid scanner device (13, 14) by producingirradiation volumes of specific shape, the irradiation being soperformed that a homogeneous dose distribution is established in eachirradiation volume; and the radiation dose is measured at various depthpositions of the phantom and compared with a reference value.
 4. Methodaccording to claim 3, characterised in that the shape of the irradiationvolumes produced by the grid scanner device (13, 14) isparallelepipedal.
 5. Method according to claim 4, characterised in thatthe irradiation is so performed by the grid scanner device (13, 14) thatthe edge lengths of the parallelepipedal irradiation volumes areapproximately 2.5 cm, 5 cm and 10 cm.
 6. Method according to claim 3,characterised in that the radiation dose is measured at the variousdepth positions of the phantom using ionisation chambers.
 7. Methodaccording to claim 3, characterised in that the various depth positions,at which the radiation dose is measured, correspond to a depth of theparallelpipedal irradiation volume of 5 cm, 12.5 cm and 20 cm.
 8. Methodaccording to claim 1, characterised in that the radiation dosedistribution of the grid scanner device (13, 14) transverse to the beamdirection of the heavy ion beam (11) is measured and evaluated. 9.Method according to claim 8, characterised in that a phantom isirradiated using the grid scanner device (13, 14) by producingirradiation areas, a plurality of irradiations being carried out usingvarious irradiation energies and various irradiation areas and, in eachcase, the radiation dose being measured perpendicular to the beamdirection of the heavy ion beam (11).
 10. Method according to claim 9,characterised in that, the various irradiation energies of the pluralityof irradiations correspond to 100 MeV/u, 150 MeV/u, 200 MeV/u, 250MeV/u, 300 MeV/u and 350 MeV/u.
 11. Method according to claim 9,characterised in that the radiation dose is measured using ionisationchambers.
 12. Method according to claim 9, characterised in that theradiation dose is measured with the aid of a verification film, which isblackened as a result of the respective irradiation, the range ofvariation in the blackening of the verification film brought about as aresult of the irradiation being evaluated for assessing the homogeneityof the radiation dose distribution transverse to the beam direction ofthe heavy ion beam (11).
 13. Method according to claim 9, characterisedin that the irradiation of the phantom for determining the homogeneityof the radiation dose distribution transverse to the beam direction ofthe heavy ion beam (11) is performed using a radiation dose ofapproximately 1 Gy.
 14. Method according to claim 1, characterised inthat the tolerance limit value for the range of variation is ±5%. 15.Method according to claim 1, characterised in that, in the region of theisocentre (10), at least one beam parameter of the heavy ion beam (11)is monitored using appropriate monitoring means, the monitoring meansprovided for monitoring the at least one beam parameter of the heavy ionbeam (11) being calibrated in accordance with a specific calibrationfactor; and the variation in the calibration factor over time ismonitored, it being concluded that the consistency of the calibrationfactor of the monitoring means over time is adequate if it does notexceed a specific tolerance limit value.
 16. Method according to claim15, characterised in that the monitoring means provided for monitoringthe at least one beam parameter of the heavy ion beam (11) compriseionisation chambers, which monitor, as the beam parameter, the particlecount of the heavy ion beam (11).
 17. Method according to claim 15,characterised in that for checking the consistency of the calibrationfactor of the monitoring means, a solid-body phantom is irradiated. 18.Method according to claim 15, characterised in that, the consistency ofthe calibration factor of the monitoring means is checked for aplurality of different irradiation energies, it being concluded that theconsistency of the calibration factor of the monitoring means over timeis adequate only if the tolerance limit value is met for each of thoseirradiation energies.
 19. Method according to claim 15, characterised inthat the tolerance limit value corresponds to a maximum variation of±3%.
 20. Method according to claim 15, characterised in that theinfluence of the particle fluence of the heavy ion beam (11) on thecalibration factor is monitored.
 21. Method according to claim 20,characterised in that a specific irradiation area of a phantom isirradiated at a specific irradiation energy and constant beam intensityand the radiation dose in the middle of the irradiation area isascertained, the discrepancy between the measured radiation dose and areference value being determined and it being concluded that there is anerror if the discrepancy is greater than ±3%.
 22. Method according toclaim 21, characterised in that the irradiation area of the phantomcorresponds to a surface area of 5×5 cm² which is irradiated atirradiation energies of 150 MeV/u, 250 MeV/u and 350 MeV/u, themonitoring units of the monitoring means being so set that nominally, asa reference value, a radiation dose of 0.2 Gy, 0.5 Gy and 1 Gy,respectively, is brought about at the measurement site.
 23. Methodaccording to claim 15, characterised in that the influence of theparticle flow of the heavy ion beam (11) on the calibration factor ismonitored.
 24. Method according to claim 23, characterised in that aplurality of irradiations are carried out using different beamintensities of the heavy ion beam (11), the nominal radiation dose beingkept constant; and the radiation dose values measured by the monitoringmeans are compared with the nominal radiation dose, it being concludedthat an error is present if, for at least one of the beam intensities,there is a departure of the measured radiation dose from the nominalradiation dose of more than ±3%.
 25. Method according to claim 15,characterised in that the dependence of the calibration factor on thebeam position of the heavy ion beam (11) is monitored.
 26. Methodaccording to claim 25, characterised in that a specific irradiationvolume is so irradiated by the grid scanner device (13, 14) that aspecific radiation dose is brought about in the centre of theirradiation volume, the departure of the radiation dose values measuredat a lateral position with respect to the centre of the irradiationvolume from the specific radiation dose being determined; and it isconcluded that an error is present if the departure is greater than ±3%.27. Method according to claim 26, characterised in that the specificradiation dose in the centre of the irradiation volume, which is in theshape of a cube having an edge length of approximately 5 cm, is 1 Gy;and the lateral positions at which the radiation dose is measured are 3cm and 6 cm from the centre of the irradiation volume.
 28. Methodaccording to claim 1, characterised in that the consistency of theradiation dose is checked by irradiating a specific irradiation volumeof a phantom by means of the grid scanner device (13, 14), the isocentre(10) being located in the centre of the front face of the phantom, andthe radiation dose occurring in the centre point of the irradiationvolume being ascertained as a reference value and compared with thesubsequently measured values of the radiation dose.
 29. Method accordingto claim 28, characterised in that it is concluded that the consistencyof the radiation dose is adequately accurate if the discrepancy betweenthe individual values of the radiation dose and the reference value ismaximally ±3%.
 30. Method according to claim 27, characterised in thatthe irradiation volume has a cube shape having an edge length ofapproximately 5 cm, the radiation dose being measured at awater-equivalent depth of 11.3 cm.